1. Field of the Invention
The present invention relates to a heat resistant stainless steel being widely used for high temperature materials in the steel manufacturing industry, the petrochemical industry, etc., and a method for coating by diffusion of aluminum, and more specifically, to a heat resistant stainless steel coated by diffusion of aluminum and the coating method thereof, in which an intermediate layer having the chromium-rich phase under an aluminide layer is formed but an interdiffusion layer is not formed.
2. Description of the Prior Art
Diffusion coating of metal is a method for obtaining the coating layer by diffusing on the surface of materials the component such as chromium, aluminum and silicon independently or more than two kinds thereof at the same time so as to protect from damage of materials against the exterior environment leading to high temperature oxidation, sulfidation, etc., to extend the span of life. Of these, one of the most widely used up to now is the aluminide coating method by diffusion of aluminum. As this method was developed to apply to the parts of aircraft engines mainly, a great deal of study for the alloy of nickel matrix and cobalt matrix was made but the optimum technology for an iron base alloy has not been established as yet.
The process for diffusion coating of aluminum is explained in the following. First, pack powder composed of a source of aluminum, activators, inert filler materials is prepared and mixed well. For a source of aluminum, aluminum powder or aluminum alloy powder is used. For an activator, halide compound enabling to form volatile gas by reacting on aluminum is used. For inert filler materials which play the role of preventing the pack powder from sintering at the high temperature, aluminina powders are used mostly. Then, the material to be coated which is well cleaned, is buried in the pack powder and heated at a non-oxidizing atmosphere. When the high temperature is attained, aluminum is reacted with an activator and thus aluminum halide gas is formed. This gas is resolved when reaching at the surface of the material to be coated, only aluminum remains on the surface of the material to be coated, and aluminum is diffused into the material to be coated, whereby intermetallic compound such as (Fe, Ni).sub.2 Al.sub.3 or (Ni, Fe)Al is formed in succession as well as an interdiffusion layer is formed concurrently at the under part thereof.
A heat resistant stainless steel having an iron as a base metal is widely used in the energy mass-consumptive industry such as petroleum, chemical and steel industries, as its high temperature strength is high, its room temperature processing is easy and it has the advantage of cheapness in comparison with the price of nickel base alloy. Recently, as the method to improve high temperature corrosion resistance of these materials, aluminum diffusion coating attracts public attention.
Using the conventional diffusion coating method for stainless steel, an aluminide layer(2) [(Fe, Ni).sub.2 Al.sub.3 or (Ni, Fe)Al] is formed outside a matrix metal(1) and an interdiffusion layer(3) composed of aluminide precipitates and a ferrite matrix is formed thereunder, as shown in FIG. 1. The interdiffusion layer can be adjusted thicker or thinner than the aluminide layer according to component of pack powder and heat treatment temperature. Meanwhile, aluminide layers are usually brittle and easily fractured when they are excessively thick. The interdiffusion layer can secure ductility as aluminide phase exists in the form of precipitates on the ductile ferrite matrix.
To make use of these properties, the composite coating method forming only an interdiffusion layer by diffusion coating of aluminum with ferrite stabilizing elements such as chromium, niobium and molybdenum is presented in the U.S. Pat. No. 4,835,010.
In this method, chromium is mainly used as the ferrite stabilizing elements. As aluminum diffusion is easily produced in the ferrite phase rather than in the austenite phase, the interdiffusion layer which aluminide is dispersed on the ferrite phase can be easily obtained by adding the ferrite stabilizing elements. In the coating method, aluminum activity is lowered to such an extent that an aluminide layer is not formed, and to coat chromium therewith, Cr-Al alloy powder alloying aluminum of 10-20% is used as a source of aluminum. And when intending to adjust aluminum activity low without adding the ferrite stabilizing elements, Ni-Al alloy powder is used. By using these methods, the coating composed of an interdiffusion layer(3) composed of aluminide precipitates and ferrite matrix can be obtained as shown in FIG. 2.
Meanwhile, the aforesaid composite coating method has the advantage of obtaining ductile coating. In the methods, however, the problem which aluminum required for forming aluminide playing the protection role with regard to high temperature oxidizing atmosphere cannot be supplied for a long time, arises because aluminum concentration of coating layer is low. Also, the methods have a problem which the material costs much, as the alloy powder of expensive elements such as chromium and nickel is used to adjust aliminum activity low when the coating is done. Further, even though cheap Fe-Al alloy can be used, as the aluminum content decreases very much to obtain the sufficiently low aluminum activity from this alloy system, the method has a problem which forming Fe-Al powder by crushing is difficult. Also, it has a problem of high costs as heat treatment should be conducted at temperatures higher than 1,100.degree. C. to suppress the formation of the aluminide layer and to deepen a diffusion depth of aluminum.
The life span of an aluminide coating is changed according to the content of aluminum and the length of time that aluminum remains in the coating. There are two processes in reducing the concentration of alunium. One is that an alumina protection film formed at high temperatures falls apart from the surface by thermal shock. The other is that aluminum of the coating exposed in the high temperature is diffused into a matrix. Consequently, extension of life span of the material and the coating is possible if the aforesaid two processes can be suppressed. To prevent aluminum depletion due to spalling of alumina, adhesion of alumina should be increased. For increasing adhesion of the protection film, there is a method of adding rare earth elements in the coating. To prevent aluminum diffusion into the matrix, a layer enabling to prevent from diffusing can be thought. Fitzer, Maurer, et al. proposed the technology of extending the life span of the coating by forming diffusion barrier composed of the main component of alloy element having low solubility with regard to aluminide when diffusion coating on a nickel base alloy is done (E. Fitzer and J. J. Maurer: Materials and Coatings to Resist High Temperature Corrosion, Applied Science, London, 1978, P253). Their studies revealed that a diffusion barrier they proposed has Cr and Ni as main components and effects of obstructing aluminum from diffusing inside are apparent. And Godlewska et al. reveal that Cr-rich precipitates which exist under the coating when aluminum diffusion coating of nickel matrix superalloy is done, obstruct aluminum from diffusing inside and thus are of help to resistivity of coating for oxidation in view of a long term (E. Godlewska and K. Godlewski, Oxidation of Metals, Vol. 22, Nos. 3/4, 1984, P117).
The present inventor made researches and conducted experiments to solve the aforesaid problems and proposed the present invention based upon the results thereof. The object of the present invention is to provide for a heat resistant stainless steel coated by aluminum diffusion and the aluminum diffusion coating method for a heat resistant stainless steel in which heat resistance and corrosion resistance are improved by burying a heat resistant stainless steel containing a large content of Ni and Cr in the appropriate diffusion coating pack powder, by coating with heat treatment, and by forming, under the aluminide layer, Cr-rich intermediate layer.